Diesel engines enjoy an advantage over gasoline engines in that the diesel engines are much more fuel efficient than gasoline engines. It is well known that the gaseous waste products, hydrocarbons, carbon monoxide and nitrogen oxides, from gasoline engines pose a serious health problem to the population at large. In addition to these gaseous pollutants, diesel engines also emit "soot" particles comprising carbonaceous solids containing adsorbed hydrocarbons and inorganic compounds or very fine droplets of condensate or a conglomerate of the two "particulates". The "particulates" referred to herein as "diesel soot" are particularly rich in condensed polynuclear hydrocarbons, some of which have been found to be carcinogenic. Owing to these factors, the United States Environmental Protection Agency (EPA) has promulgated strict standards to minimize the discharge of diesel soot from automotive sources, including buses and trucks, into the atmosphere. California has also enacted regulations regarding emission of diesel soot from stationary sources.
Several approaches have been proposed to try to solve the diesel emission problem. Among these are: (1) electrostatic precipitators; (2) paper filters; (3) ceramic filters; (4) metal mesh filters; and (5) engine modifications. Electrostatic precipitators are too bulky and require too much energy to operate and are therefore impractical. Similarly, paper filters require frequent replacement and are extremely bulky. Engine modifications are capable of reducing the soot emissions, but not to the point where all vehicles can meet all the emission standards. The reason for this is that modifications which reduce the soot emissions generally increase the nitrogen oxides emissions or reduce the practical operation of the engine.
Ceramic and metallic filters have proven to be the best technology available to deal with this problem. The literature also shows that ceramic filters are preferred over metallic filters because the ceramic filters are apparently more durable. Ceramic filters have been described in the prior art and can be divided into two categories: (1) foam type and (2) honeycomb wall-flow type. Ceramic foam filters have been described in U.S. Pat. No. 4,083,905 and Society of Automotive Engineering Paper #830082. This type of filter is prepared by depositing a ceramic material onto an organic sponge and sintering said sponge at a high temperature to burn out the organic sponge material.
The honeycomb wall-flow type filters are very similar to the honeycomb substrates used as catalyst structural supports for gasoline engine pollution control applications, except that alternate flow channels are closed on each face of the substrate. The channels are plugged in such a manner that a channel open on one face is closed at the opposite face. Such filters are called "wall-flow filters" because the exhaust flows down a channel and must go through the walls of the channels, which are macroporous, in order to exit. These filters are described in U.S. Pat. Nos. 4,329,162, 4,340,403, 4,364,760 and 4,423,090. These wall-flow filters have been used more extensively than the foam filters because the wall-flow filters more efficiently trap the diesel soot.
Another type of filter which can be used is a conventional automotive catalyst carrier. That is, ceramic honeycomb monolithic carriers or particulate, e.g., spheres, pellets, and extrudates, carriers. The efficiency of these filters is not very high, but they may be used in applications where only a small reduction in particulate emissions is needed to meet the EPA standards.
The biggest drawback to these filters is that the diesel soot accumulates, clogging the filter, thereby causing an undesirable backpressure on the engine. The reason for this accumulation is that diesel soot ignites at about 650.degree. C., but the maximum exhaust temperature in a diesel vehicle is only about 300.degree.-400.degree. C. Therefore, the diesel soot continues to build up and causes excessive backpressure on the engine which results in a decrease in fuel economy and eventually may cause damage to the engine.
It should be noted that conventional automotive catalyst carriers do not build up as much backpressure as the metal mesh and ceramic filters, because of their design and low trapping efficiency. To alleviate the backpressure problem, the diesel soot must be burned off. This is referred to as trap regeneration. There are two ways known in the art to burn or ignite the diesel soot collected on these filter traps. First, an external means of heat can be applied to the filter so that the temperature of the filter is raised high enough to initiate soot combustion and thus regenerate the filter. Second, the filter can be coated with a catalytic element that will lower the combustion temperature of the diesel soot.
The first approach has many disadvantages including: (1) reduction of fuel economy; (2) complexity of the control system; and (3) reliability of the overall system. In contrast, the second approach is much simpler and more reliable. The major problem with the second approach is developing a catalytic composite which lowers the ignition temperature of the diesel soot so that combustion of the diesel soot occurs continuously during normal operating conditions. Even though a catalytic composite may not be active enough to allow continuous combustion of the soot, it may be used in conjunction with external means of regenerating the filter.
In addition to igniting the diesel soot, most catalytic composites will also convert the sulfur oxides in the exhaust of sulfates or sulfuric acid. Although this problem is present in gasoline powered engines, it is particularly troublesome in diesel applications for two reasons. First, diesel fuel typically contains at least ten timess more sulfur than gasoline fuel. Second, the low temperature of the diesel exhaust facilitates the production and storage of sulfates and sulfuric acid. During high temperature modes such as trap regenerations, the sulfates and sulfuric acid are released and contribute to the total particulate emissions.
It is recognized that noble metals, especially platinum, can oxidize gaseous hydrocarbons but also promote the conversion of sulfur oxides to sulfates. U.S. Pat. No. 4,617,289 claims to solve this problem by adding large amounts of vanadium oxide (V.sub.2 O.sub.5) to minimize the sulfate formation.
The prior art teaches that platinum or other noble metals are not a preferred metal for combusting diesel soot unless a promoter is used. For example, see U.S. Pat. No. 4,617,289 and references therein. Additionally, other patents teach that platinum should be used only for converting the gaseous hydrocarbon and other elements such as chromium, silver, etc. are to be used for igniting the soot. For example, U.S. Pat. No. 4,303,552 teaches the use of platinum and a bulk component selected from the group consisting of an element of the first transition series, silver and hafnium deposited on an inorganic oxide, preferably alumina.
Further, U.S. Pat. Nos. 4,515,758 and 4,588,707 teach the use of rhenium plus substances such as lithium oxide, copper chloride, vanadium oxide and optionally a noble metal. Again the noble metal is used only for treating the gaseous emissions. These patents also teach that the soot burning elements, i.e. rhenium, lithium oxide, etc. are deposited on an inorganic oxide support such as alumina, titania, etc. It is also known that supports such as titania or zirconia have sulfur reistant properties. For example, see U.S. Pat. No. 4,350,613.
In contrast to the prior art the present invention provides a catalytic composite effective in reducing the ignition temperature of diesel soot, which catalyst consists of a sulfur resistant refractory inorganic oxide such as titania, zirconia, etc. having dispersed thereon at least one catalytic element selected from the group consisting of Pt, Pd and Rh. This catalytic composite may be deposited on a number of types of diesel soot filters. The difference between this catalytic composite and that of the prior art is that the catalytic composite of the present invention uses only a Pt, Pd or Rh component to combust the soot, whereas the prior art catalytic composites use a bulk component such as chromium, silver, etc. to combust the soot. Also, the catalytic composite of the present invention uses a sulfur resistant support such as titania, which does not store as much sulfate as prior art supports. The catalytic composite of the present invention also ignites the soot at lower temperatures than prior art catalysts.
The present invention also provides a process for reducing the ignition temperature of diesel soot comprising contacting an exhaust gas from a diesel engine with the catalytic composite described above.